Despite its apparent simplicity, LNG ship unloading poses various significant challenges in several economic and technical aspects. For example, when LNG is unloaded from an LNG ship to a storage tank, LNG vapors are generated in the storage tank due to, among other factors, volumetric displacement, heat gain during LNG transfer and pumping, boil-off in the storage tank, and flashing (due to the pressure differential between the ship and the storage tank). In most cases, these vapors need to be recovered to avoid flaring and pressure buildup in the storage tank system.
Moreover, LNG unloading docks and LNG storage tanks are often separated by relatively large distances (e.g., as much as 3 to 5 miles), which frequently causes significant problems to maintain LNG in the transfer line at cryogenic temperatures (i.e., −255° F. and lower). Worse yet, additional heat is introduced into the LNG by the transfer pumps as the ship unloading pumping horsepower is relatively high to overcome pressure losses due to the long distance between the ship and the storage tanks. As a consequence, large amounts of LNG vapor are formed that must be further processed.
Furthermore, the LNG storage and unloading system must also be maintained at a stable pressure. To that end, a portion of the vapor coming from the storage tank is typically compressed by a vapor return compressor and returned to the ship to make up for the displaced volume. In such configurations, a dedicated vapor return line is required which adds significant cost to the LNG receiving terminal. The excess vapor from the storage tanks is compressed to a sufficiently high pressure by a boil-off gas compressor for condensation in a vapor condenser that utilizes the refrigeration content from the LNG sendout from the storage tank. As relatively large volumes of vapor are handled by such compressors, currently known compression and vapor absorption systems require significant energy and operator attention, particularly during transition from normal holding operation to ship unloading operation. During normal holding operation, the LNG transfer line generally remains stagnant, which leads to an increase in temperature and thermal stress on the transfer line. Alternatively, vapor control can be implemented using a reciprocating pump in which the flow rate and vapor pressure control the proportion of cryogenic liquid and vapor supplied to the pump as described in U.S. Pat. No. 6,640,556 to Ursan et al. However, such configurations are often impractical and fail to eliminate the need for vapor recompression in LNG receiving terminals.
Alternatively, or additionally, a turboexpander-driven compressor may be employed as described in U.S. Pat. No. 6,460,350 to Johnson et al. Here the energy requirement for vapor recompression is typically provided by expansion of a compressed gas from another source. However, where compressed gas is not available from another process, such configurations are typically not implemented. In still other known systems, methane product vapor is compressed and condensed against an incoming LNG stream as described in published U.S. Pat. App. No. 2003/0158458. While such systems increase the energy efficiency as compared to other systems, various disadvantages nevertheless remain. For example, vapor handling in such systems requires costly vapor compression and is typically limited to plants in which production of a methane rich stream is desired.
In yet another system, as described in U.S. Pat. No. 6,745,576, mixers, collectors, pumps, and compressors are used for re-liquefying boil-off gas in an LNG stream. In this system, the atmospheric boil-off vapor is compressed to a higher pressure using a vapor compressor such that the boil-off vapor can be condensed. While such a system typically provides improvements on control and mixing devices in a vapor condensation system, it nevertheless inherits most of the disadvantages of known configurations as shown in Prior Art FIG. 1.
Thus, most of the currently known processes and configurations for LNG ship unloading and regasification require vapor compression and absorption that are typically energy inefficient. Therefore, there is still a need for improved configurations and methods for vapor handling in LNG unloading and regasification terminals.